C Programming: Extract Data from Excel Sheets Easily

Working with data from Excel spreadsheets can often be a cumbersome task, especially if you need to manipulate or analyze it programmatically. However, C, with its efficiency in handling complex data structures and its wide array of libraries, offers an excellent way to automate this process. In this comprehensive guide, we'll delve into how to extract data from Excel sheets using C, touching on various techniques, libraries, and best practices to make your data extraction journey smoother.

Understanding the Basics of Excel File Formats

Excel files come in several formats, the most common being:

• .xls: The legacy format, primarily used before Excel 2007.
• .xlsx: The newer, XML-based format, which offers better performance and data recovery.
• .xlsm: Similar to .xlsx but for macro-enabled workbooks.

These formats have different structures, which will influence how you interact with them:

• .xls: Uses Compound File Binary Format (CFBF), a complex format that stores data in a binary stream.
• .xlsx and .xlsm: Uses the Open Packaging Convention (OPC), based on ZIP compression and XML to store workbook data.

Excel File Structure

Understanding the internal structure can help with parsing:

Libraries for Reading Excel Files in C

To extract data from Excel sheets, you'll need a library that can parse these file formats. Here are some popular options:

libxl

libxl is a library that can read/write Excel files (.xls and .xlsx). It’s straightforward and doesn’t require extensive Excel knowledge:

#include 

int main() {
    BookHandle book;
    SheetHandle sheet;
    unsigned int i, rows, cols;

book = xlCreateBook();  // This opens a new workbook, which we'll need to populate from an existing file.

if (xlBookLoad(book, "myfile.xlsx")) {
    sheet = xlBookGetSheet(book, 0);
    rows = xlSheetRowCount(sheet);
    cols = xlSheetColCount(sheet);

    for (i = 0; i < rows; ++i) {
        const char* cellValue = xlSheetReadStr(sheet, i, 0, 0);
        printf("Row %d: %s\n", i, cellValue);
    }
}

xlBookRelease(book);
return 0;


}

SimpleSpreadsheet

For those needing to handle .xlsx files specifically, SimpleSpreadsheet is a lightweight alternative:

#include 

int main() {
    spss_workbook *workbook = spss_open_workbook(“example.xlsx”);
    spss_sheet *sheet = spss_workbook_get_sheet(workbook, 0);

for(int i = 0; i < spss_sheet_row_count(sheet); i++) {
    const char *value = spss_sheet_get_cell_value(sheet, i, 0);
    printf("Row %d: %s\n", i, value);
}

spss_close_workbook(workbook);
return 0;


}

⚠️ Note: Remember to compile your program with the respective library's headers and link against their libraries. Refer to the library's documentation for specific commands.

Advanced Data Extraction Techniques

Beyond the basic reading of cells, here are some advanced techniques:

Dynamic Data Extraction

• Identify regions with data changes or specific criteria through dynamic range reading.
• Use formulas or specific conditions to extract data that meets particular requirements.

Multiple Sheets

• Manage workbooks with multiple sheets by iterating through each one to extract data.
• Handle sheets with different formats and structures within the same file.

Date and Time Handling

Excel stores dates as serial numbers, which you need to convert back to a readable format:

time_t epoch_time = (time_t)(sheetReadNum(sheet, row, col) - 25569) * 86400; // 25569 is the date origin for Excel
struct tm *date = gmtime(&epoch_time);
char buffer[80];
strftime(buffer, sizeof(buffer), “%Y-%m-%d”, date);
printf(“Date: %s\n”, buffer);

Error Handling and File Management

Proper error handling is crucial when working with files:

• Ensure that the Excel file exists and is not corrupted.
• Handle cases where the library can’t read specific cell types or when the structure of the Excel file is unexpected.
• Manage file permissions, especially when trying to read from protected or locked files.

📌 Note: Always validate and sanitize input when dealing with external files to avoid security vulnerabilities.

This guide has provided you with the tools and techniques to effectively extract data from Excel sheets using C. From understanding file formats to employing specialized libraries for parsing and reading data, we've covered a broad spectrum of approaches that can make your data extraction process more efficient and less error-prone.

By leveraging libraries like libxl or SimpleSpreadsheet, you can now write programs to automate data extraction, ensuring that your applications are more dynamic and adaptable to varying data structures within Excel files. Whether you're dealing with large datasets, complex sheets with multiple formats, or need to perform specific data manipulations, these methods allow you to handle Excel data with precision.

Incorporating best practices such as error handling, understanding Excel's date encoding, and managing workbook sheets ensures that your code is robust and can adapt to real-world scenarios where Excel files might not always be perfectly formatted. This knowledge not only saves time but also significantly improves the quality of data analysis and processing in your programming projects.

What’s the difference between .xls and .xlsx file formats?

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.xls is the legacy format for Microsoft Excel, used in versions before 2007, employing the Binary Interchange File Format (BIFF). .xlsx is a newer, XML-based format introduced with Excel 2007, using Open Packaging Convention (OPC) and Zip compression, offering better performance and file recovery features.

Can I read .xlsm files with libxl?

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Yes, libxl supports reading and writing .xlsm files, which are similar to .xlsx files but enable macros.

How do I handle Excel date values in C?

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Excel stores dates as serial numbers with January 1, 1900, as the base date. You need to convert these numbers by subtracting 25569 (the date origin for Excel), then multiply by 86400 to get epoch time, which you can then convert to a readable date format using time libraries like time.h.

What libraries can I use to read Excel files in C?

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Some popular libraries for this purpose include libxl for both .xls and .xlsx formats, SimpleSpreadsheet specifically for .xlsx, and Excel-RW which allows reading and writing to Excel 2007 and later formats.

How can I handle errors when reading Excel files?

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Implement thorough error checking by ensuring the file exists, is not corrupted, checking for correct permissions, and handle cases where the library can’t read specific cell types. Validate and sanitize input to avoid security issues.